(1) Field of the Invention
The present invention relates to a microwave transmission line, and more particularly relates to a microwave transmission line formed on a high-resistivity silicon substrate.
(2) Description of Related Art
Microwave radio communication apparatuses and microwave radio communication terminals are being used in many fields, notably for consumer use. Group III-V compound semiconductors are often used for semiconductor substrates on which microwave front-end circuits of such radio communication apparatuses are formed. The reason for this is not only that active elements formed on compound semiconductor substrates each have an excellent high-frequency characteristic but also that such semi-insulating substrates can facilitate providing low-loss microwave transmission lines. On the other hand, Group III-V compound semiconductor substrates have the following demerits: their prices are high; since an intermediate-frequency (IF) stage and a signal processing part of such a radio communication apparatus are formed on a semiconductor substrate usually made of silicon (Si), the IF stage and the signal processing part cannot be integrated with a microwave front-end circuit; the yield of elements formed on the Group III-V compound semiconductor substrate tend to be lower than that of elements formed on a silicon substrate; and Group III-V compound semiconductor substrates each have a low thermal conductivity. These demerits have promoted the growth of the needs for forming microwave front-end circuits on silicon substrates.
A silicon substrate fabricated by a typical Czochralski (CZ) method has a resistivity of 100 Ωcm or less and is thus inadequate to a substrate for a microwave transmission line. On the other hand, a high-resistivity p−-type silicon substrate fabricated by a floating zone (FZ) method can provide a high resistivity of 2 kΩcm or more. In theory, as long as the substrate serving as a medium through which a signal electric field of a microwave transmission line propagates has a resistivity of the similar value, a low-loss microwave transmission line should be able to be constructed which is close to a semi-insulating substrate of gallium arsenide (GaAs).
In spite of this, a problem actually arises in that a charge inversion layer is produced on a silicon substrate so that a high-resistivity p−-type silicon substrate has a significantly reduced resistivity in vicinity of its top surface.
A natural oxide film is produced on the top surface of a silicon substrate, resulting in the changed physical properties of silicon. Therefore, a silicon oxide film is generally previously formed, as a protective film, on the silicon substrate. In this case, electrons, minority carriers, are stored at the interface between the p−-type silicon substrate and the silicon oxide film and in the vicinity thereof. The reason for this is as follows: positive interfacial charges are produced at the interface between the silicon oxide film and the silicon substrate; positive charges are produced due to the surface level obtained by impurities of the silicon substrate; and the silicon oxide film itself becomes positively charged by impurities, i.e., sodium (Na) ions. It is considered that the above-mentioned storage of electrons will provide, in the vicinity of the top surface of the p−-type silicon substrate, a charge inversion layer having a small thickness not more than 0.03 mm but having a reduced resistivity of approximately 0.03 Ωcm. This makes it difficult to realize a low-loss microwave transmission line, even with the use of a high-resistivity p−-type silicon substrate.
For example, Document 1 (A. C. Reyes, et al., “Coplanar Waveguides and Microwave Inductors on Silicon Substrates”, IEEE Trans. on Microwave Theory and Tech., vol. 43, No. 9, September, 1995) discloses the following configuration. A strip conductor of a microwave transmission line is arranged with a barrier metal interposed between a p−-type silicon substrate and the strip conductor without providing a silicon oxide film on the substrate.
Furthermore, Document 2 (Y. Wu, et al., “SiO2 Interface Layer Effects on Microwave Loss of High Resistivity CPW Line”, IEEE Microwave and Guided Wave Letters, Vol. 9, No. 1, January, 1999) discloses the following configuration. A low-loss microwave transmission line similar to a gallium arsenide substrate is achieved by removing a part of a silicon oxide film other than that located under the microwave transmission line to restrain a charge inversion layer from being produced in a part of the microwave transmission line to which an electric field is applied. However, for both Documents 1 and 2, long-term stability and long-term reliability might not sufficiently be achieved, because p−-type silicon substrates in both cases are exposed.
Furthermore, Document 3 (Japanese Unexamined Patent Publication No. 8-316420) discloses the following technique: a positive charge layer is formed on the top surface of a p−-type silicon substrate by previously implanting boron (B) ions into the substrate, and negative charges stored in the p−-type silicon substrate are cancelled by positive space charges arising from impurity (Na) ions contained in a silicon oxide film, thereby restraining a charge inversion layer from being produced.
With the method of Document 3, an annealing process is required for activating the ion-implanted boron and requires the heating of the substrate at 800° C. or more. If a transistor, a diode or the like has been formed on the p−-type silicon substrate, doped impurities might be thermally diffused into the substrate or thin films already formed might peel off. Furthermore, since ions need be implanted into the substrate at least before the formation of at least the silicon oxide film, this makes it difficult to optimize the dosage of impurity ions while monitoring the thickness of the produced charge inversion layer.